Don't know about your harney, but some coaches had foam backing on the ceiling material. As the material aged the foam dried out broke down and came loose from the ceiling.if that is what is happening to yours I doubt you will have much luck reglueingit and make it look like it used to. I know some who put wood strip across the material to hold it up, but depends on material and how and why it is letting go.
Howard,Connie,& Bella,
One spoiled schnauzer
2007 Newmar KSDP
4dr Jeep Wrangler